Recently, we described a new process for making polyetherester resins from polyethers (see U.S. Pat. No. 5,319,006). The process reacts a polyether with a cyclic anhydride (such as maleic anhydride) in the presence of a Lewis acid catalyst. The anhydride inserts randomly into carbon-oxygen bonds of the polyether to generate ester bonds in the resulting polyetherester resin. The polyetherester resin is then combined with a vinyl monomer, preferably styrene, and is cured to produce a polyetherester thermoset.
We later found that, in addition to Lewis acids, protic acids that have a pKa less than about 0 and metal salts thereof will catalyze the insertion of an anhydride into the polyether to produce a polyetherester (see copending application Ser. No. 08/220,149, filed Mar. 30, 1994). We also discovered that these strong protic acids and their metal salts will catalyze the insertion of a carboxylic acid into a polyether (see copending application Ser. No. 08/228,845, filed Apr. 18, 1994).
The ability to prepare polyetheresters by random insertion of anhydrides and carboxylic acids into polyethers provides a valuable way of making many unique polyetherester intermediates. These polyetheresters often have favorable performance characteristics compared with polyesters made by conventional esterification processes. Unfortunately, the insertion process does not work particularly well with high-melting aromatic dicarboxylic acids (such as isophthalic and terephthalic acids). Aromatic dicarboxylic acids are commonly incorporated into conventional unsaturated polyester resins to impart good mechanical properties and chemical resistance to thermosets made from the resins.
As we described in copending application Ser. No. 08/228,845, carboxylic acids, including aromatic dicarboxylic acids, can be inserted in one step into polyethers using strong protic acids or their metal salts as catalysts. Examples 2 and 5 of that application illustrate the insertion process with 20 wt. % isophthalic acid. The examples show that it is possible to make polyetheresters having high aromatic ester content by a single-step insertion process.
The single-step process illustrated by those examples has some disadvantages compared with the process of the invention when such high levels of aromatic dicarboxylic acids are used. First, relatively high catalyst levels (typically 1 wt. % or higher) are needed for the single-step insertion process to give satisfactory reaction rates. Second, the yield of polyetherester resin obtainable is somewhat less than desirable. Third, resin consistency is difficult to achieve with the single-step process. As the comparative examples (See C12-C14) in this application illustrate, the single-step insertion process may be too slow at desirable catalyst levels of less than about 0.5 wt. %. The reactions can be incomplete even after several days of heating at elevated temperature, and the products often become discolored.
Thus, while at least about 10 wt. % of aromatic dicarboxylic acid content is desirable in polyetheresters to give them good mechanical properties and chemical resistance, the single-step insertion process is not completely satisfactory for making these products.
Ordinary esterification procedures can be used to make polyetheresters. For example, one can react a low molecular weight polyol, a glycol, maleic anhydride, and isophthalic acid in a single-step cook to make a polyetherester. Unfortunately, thermosets made from this type of product often lack the desirable physical and mechanical properties available from polyetheresters made by an insertion process. We believe that the relatively slow reactivity of isophthalic acid compared with that of maleic anhydride in the ordinary esterification process adversely impacts the product.
A valuable process would capitalize on the improved properties available from polyetheresters made by an insertion process, but would also facilitate the inclusion of more than about 10 wt. % of recurring units of an aromatic dicarboxylic acid in the polyetherester. Ideally, the process would be easy to perform at low catalyst levels, would give consistent resins, and would not require excessively long reaction times or high temperatures.